A provocative new hypothesis suggests that Yosemite Valley was carved by an ancient volcano and a disappearing river, both of which have long since eroded away.
Geologists have long debated why Yosemite Valley is so deep, with walls that tower up to 4,000 feet (1,219 meters) above the valley floor. The prevailing explanation is that in the last 10 million years, the Sierra Nevada mountains of California experienced a period of uplift, steepening their slope and causing the rivers to flow more quickly and erode more quickly into the granite around them.
But a new study, published April 3 in the journal Geosphere, suggests uplift wasn’t the real reason Yosemite exists. Instead, said study co-author Manny Gabet, a geomorphologist at San Jose State University, the landscape of Yosemite and the surrounding Sierras is better explained by a long-vanished river.
Millions of years ago, this river would have increased the flow to the modern day Merced River and Tenaya Creek, which would have then had enough power to slice through the Sierras to create Yosemite Valley and nearby Tenaya Canyon.
“At some point in time,” Gabet told Live Science, “there was a big river here. And now that river is gone.”
Yosemite mysteries
Geologists agree that in the last 2 to 3 million years, Yosemite was under a glacier that helped deepen the valley. But they also believe that this glacier filled a pre-existing deep valley, said Kurt Cuffey, a geologist at the University of California, Berkeley, who was not involved in the new research.
“So why did the canyon form in the first place?” Cuffey said.
There are a lot of faults on the east side of the Sierra Nevada that likely would have caused the mountains to rise and get steeper, Cuffey told Live Science. But geologists can’t say how much higher the mountain range got, or if it was high enough to substantially increase the erosive power of the rivers. It’s a controversial topic, he said.
Uplift also doesn’t explain three odd observations, Cuffey said. The first is that Tenaya Canyon, a steep and treacherous canyon that runs from Tenaya Lake into Yosemite Valley, is way too deep to have been cut by the stream that runs through it today, Tenaya Creek. “It’s just a really small river,” Gabet said. “You can jump across it. The mystery is, how did this tiny creek cut through thousands of feet of very resistant, very massive granite?”
The second mystery is that in California’s Central Valley, where the Merced River spills out of Yosemite and creates a fan-shaped layer of sediment it has carried from the mountains, there are huge deposits of volcanic rock that shouldn’t be there. “You’ve got 8 cubic miles [33.3 cubic kilometers] of volcanic sediment deposited in the Central Valley by the Merced River, but you can’t find a scrap of these volcanic rocks,” in the area around the river, Gabet said.
The third mystery has to do with the uneven shape of the valley cut by the Tuolumne River just north of the Merced, Cuffey said. This valley is much larger on one side than the other. It’s a relatively subtle point to a non-geologist, but “that needs an explanation,” he said.
Lost river
Gabet’s hypothesis harkens back to 5 to 10 million years ago, when a chain of volcanoes had buried the northern Sierra Nevada in huge mudflows, creating a gently sloping volcanic plain with only a few mountain peaks poking out of it. These deposits are still seen north of Yosemite, but not in the area around the Merced River.
“I realized these volcanic rocks that had been transported by the Merced River must have come from this chain of volcanoes,” Gabet said.
The peaks of such a volcanic chain would have been drained by a large, now lost river, he said. This river would have flowed from now-vanished volcanic slopes north of where the National Park is today and then gushed into the ancient Merced and Tenaya Creek, enabling them to carve out Yosemite Valley and Tenaya Canyon.
The influence of this river would have made both the Merced and Tenaya Creek much larger than today’s relative trickle — so large that they could have cut down the canyons. The drainage patterns from this ghost river would also explain the lopsided topography around the Tuolumne River , Cuffey said. Finally, the river would have carried the volcanic rock now found in the Central Valley down from the northern Sierra Nevada, a journey that is hard to explain otherwise.
The river and volcano would have themselves eventually eroded to nothing, so there is no way to check if they ever existed. One of Gabet’s students is now working on a project to try to recreate the ancient topography of the Sierra Nevada to better understand how the geology of the mountains evolved and perhaps shed more light on the possibility.
“He’s got a really interesting thing going,” Cuffey said of Gabet. “I really don’t know if it’s true or not at this point, but it’s a great hypothesis that we should think about.”
